All of this goes back to a fix for issue 1961 made in November 2012 (see
https://www.mobileread.com/forums/sho...d.php?t=206193)
Now that Sigil is back in development, maybe the contemplated enhancements can be picked up again?
Or at least make the appearance of the sigil_toc_id configurable in preferences so as to mimic the pre-issue-1961-fix behavior, so as to avoid merging all files into one.